It is the conductor's job to lead the orchestra, band or choir when performing a piece of music in a concert or in a rehearsal.
It is also the conductor's job to make sure the music is of a high standard, and therefore he will have to spend many hours rehearsing his orchestra behind the scenes, picking out mistakes and making improvements to the performance.
On stage, the conductor is silent, but uses a combination of his face, his hands, his body and his conducting baton (a long white stick) to communicate to the performers exactly how he would like the music to be played: he must show whether the music is fast or slow, loud or quiet, smooth or bouncy as well as having to cue in different instruments of the orchestra when it is their time to play.